Prospects of 3D mapping of the Galactic Centre clouds with X-ray polarimetry

Abstract

Despite past panchromatic observations of the innermost part of the Milky Way, the overall structure of the Galactic Centre (GC) remains enigmatic in terms of geometry. In this paper, we aim to show how polarimetry can probe the three-dimensional position of the molecular material in the central ∼100 pc of the GC. We investigate a model where the central supermassive black hole Sgr A * is radiatively coupled to a fragmented circumnuclear disc (CND), an elliptical twisted ring representative of the central molecular zone (CMZ), and the two main, bright molecular clouds Sgr B2 and Sgr C. 8-35 keV integrated polarization mapping reveals that Sgr B2 and Sgr C, situated at the two sides of the CMZ, present the highest polarization degrees (66.5 and 47.8 per cent, respectively), both associated with a polarization position angle ψ = 90• (normal to the scattering plane). The CND shows a lower polarization degree, 1.0 per cent with ψ = −20.• 5, tracing the inclination of the CND with respect to the Galactic plane. The CMZ polarization is spatially variable. We also consider a range of spatial locations for Sgr A * and the reprocessing media, and investigate how the modelled three-dimensional geometry influences the resulting GC polarization. The two reflection nebulae are found to always produce high polarization degrees ( 10 per cent). We show that a 500 ks observation with a broad-band polarimeter could constrain the location and the morphology of the scattering material with respect to the emitting source, revealing the past activity of Sgr A * .

Reflection nebulae in the Galactic center: soft X-ray imaging polarimetry

“…In their model, the Sgr B2 cloud is expected to produce a high polarization degree associated with a direction of Article published by EDP Sciences A19, page 1 of 7 polarization normal to the scattering plane. A more elaborate investigation has been undertaken in Marin et al (2014), where we produced 8−35 keV polarization maps of the GC. We avoided the soft X-ray energies, since past X-ray observations (Koyama et al 1986(Koyama et al , 1989Sidoli & Mereghetti 1999) have revealed a diffuse plasma emission angularly superimposed to the X-ray emission of the molecular clouds.…”

The Growth of Interest in Astronomical X-Ray Polarimetry

“…In the latter case, the very low X-ray fluxes, coupled with relatively low or inexistent polarization from galaxy clusters, naturally explains why the publication rate in this field is the lowest. Until the advent of extra-sensitive X-ray polarimeters, it is not mandatory to explore in greater detail the expected polarization of quiescent galaxies (save the Milky Way, where important and feasible observations await us [22][23][24]). Several subcategories are highlighted: the black solid line represents general publications on X-ray polarimetry (reviews, historical notes); violet stands for publications concerning theory, instrumentation, and satellites, red for papers on solar, stellar, and exoplanet science; orange for papers on objects dominated by strong gravity effects (e.g., black holes); yellow for objects dominated by strong magnetic fields (e.g., neutron stars); the green solid line stands for papers related to galaxies (e.g., galaxy clusters or the Milky Way).…”

Shadow size for the supermassive black hole at the Galactic Center

“…The role of strong-gravity retro-lensing in polarized signal has been discussed in paper 11 , and the specific observational signatures of polarisation in Galactic Center flares have been examined in papers 12,13 . Promising future prospects of X-ray polarimetry of molecular clouds surrounding Galactic Center have been recently discussed in paper 14 in the context of past accretion activity of the supermassive black hole.…”